The present invention relates to an electrically conductive sample support-mounting for secondary ion mass spectrometry analysis in which the sample to be analyzed is bombarded in a vacuum with an ion or neutral particle beam. The sample is eroded at the impinged location, and the secondary ions resulting during the erosion are analyzed in a mass spectrometer. Here, an atomic beam or molecular beam is to be understood as a neutral particle beam.
The analysis method of secondary ion mass spectrometry (SIMS) is very sensitive. It can analyze traces of impurities in a material down to the ppm-range, in some instances, down to ppb-range. For this reason SIMS possesses great significance for the examination of semiconductor crystals. For example, it is highly useful for the measurement of the concentration of an ion-implanted doping material in dependence upon the sample depth. In addition, this technique is also applicable to general trace analysis.
For the attainment of maximum detection sensitivities it is necessary to detect contributions to the measuring signal exclusively from the center region of the eroded layer, the socalled sputtering crater, and to keep all secondary ions from the rim of the crater or its surroundings away from the measurement.
It is known (see the article of K. Wittmaack in Appl. Phys. 12, pp. 149-156 (1977), incorporated herein by reference), to scan the ion beam over or impinge it upon a square surface of e.g. 1 mm.sup.2 and to thereby generate an erosion crater with a flat bottom. In order not to jointly measure signal contributions from the rim of the crater, the measurement detection (pulse counting) is electronically interrupted as soon as the ion beam leaves a center region of the crater ("electronic gate"). Rapid neutral particles which arise in the ion beam and do not participate in the beam scanning (because they are not deflectable), can however not be prevented by the "electronic gate" from triggering signal contributions from the crater rim or from the surroundings.
As can be learned from an article of K. Wittmaack and J. B. Clegg from Appl. Phys. Lett. 37 (3), pp. 285-287 (Aug. 1, 1980) incorporated herein by reference, such neutral particles can be prevented to a substantial proportion from striking the sample through bending of the ion beam.
However, the following interference components can be eliminated neither by the electronic marginal blocking ("gate") nor by an ion beam bend or curvature:
1. rapid neutral particles which are created after the ion beam bend;
2. ions from far-reaching tails of the ion beam (dependent upon the focusing conditions); and
3. electrons from an electron beam simultaneously directed to the sample for the purpose of avoiding charging of the sample due to the ion beam ("compensation" of charges in the case of insulators and semiconductors).
The particle types cited herein become interfering components impairing the detection sensitivity of impurities (dopant atoms inter alia) in that they trigger undesired signal components from the region of the crater rim or the surrounding regions of the crater. Similar interfering effects also occur in the case of application of a neutral particle beam.